This invention relates to magnetizing magnets for use with high temperature superconductors (HTSCs).
HTSCs have a critical (transition) temperature, above which they cease to be superconducting, of less than 100.degree. K. and in many cases below the 77.degree. K. boiling point of nitrogen. Some of these HTSCs are magnetizable and behave like permanent magnets below their critical temperatures. One such is melt-processed single crystal yttrium barium copper oxide, Y--Ba--Cu--O (M-P YBCO), typically having the composition (Y Ba.sub.2 CU.sub.3 O.sub.7-x).
Use of M-P YBCO involves a number of problems, including the need to be able to generate the fields needed to magnetize it, and the fact that if, for any reason, the material warms up above its critical temperature it will lose all its magnetization. Then, after re-cooling, it requires further magnetization.
The problem of magnetizing the material is that it must be cooled to below 90.degree. K. (the critical temperature of YBCO) and ideally to below 50.degree. K. (as the flux it traps increases markedly as its temperature is reduced). In practice, a block of material needs to be held inside a vacuum container as primary insulation supported on insulating struts with sufficiently low conductivity to minimize heat conduction to the block, but with a necessary material strength to support the forces its enormous potential magnetization could cause to act on it in the presence of an external magnetic field. A means of keeping the material cold, typically using a good thermal conductor in contact with it which can be cooled through an appropriate low thermal resistance connection to the cold head of a refrigerator, is also needed and requires insulation. Such insulation and support tends to involve several centimeters of space (perhaps 2 to 4 centimeters) around the block of material.
Such a magnet could be used to provide, or to supplement, the main magnetic field of a magnetic resonance imaging magnet. While such a magnet would have advantages in terms of the strength of field generated in terms of its size, it would suffer the disadvantage of the problems which would be caused if its refrigerator failed and it warmed up to above its critical temperature, which could take from a few minutes to a couple of hours.
In particular, it would be desirable for the magnetizing magnet needed to re-magnetize it to be transportable so that it could be brought to the relevant imaging apparatus, perhaps one of several in a hospital, in order to allow re-magnetization to take place.
However, the bulk of the HTSC in its cryostat, allied to the potentially very high fields needed (up to 8 to 10 Tesla (T)), means that the magnetizing magnet would have to be very large and have a potentially significant fields spread. This would make it very difficult to locate and could make it impracticable to be transportable for re-magnetization of devices in-situ.